Avionics chassis

ABSTRACT

An avionics chassis comprises a housing having a substantially thermally non-conductive frame comprising a composite of carbon fibers laid up in an epoxy matrix. The housing also includes at least two walls, at least one of which is a thermally conductive wall comprising a composite of carbon fibers in a carbonized matrix, and a plurality of spaced, thermally-conductive, card rails provided on the at least two walls. The at least two walls are mounted to the frame in opposing relationship such that corresponding card rails on the walls define an effective slot therebetween in which a printed circuit board may be received and the card rails and the at least one thermally conductive wall form a thermally conductive path from the interior to the exterior.

GOVERNMENT LICENSE RIGHTS

This invention was made with Government support under Purchase Order No.4CC1766 awarded by Department of the Air Force, Air Force ResearchLaboratory. The Government has certain rights in this invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to patent application Ser. No. 12/487,784,entitled Avionics Chassis, patent application Ser. No. 12/487,797,entitled Avionics Chassis, and patent application Ser. No. 12/487,834,entitled Avionics Chassis, filed concurrently herewith.

BACKGROUND OF THE INVENTION

Contemporary aircrafts use avionics in order to control the variousequipment and operations for flying the aircraft. The avionics may bestored in an avionics chassis, which performs several beneficialfunctions, some of which are: electrically shielding the avionics fromelectromagnetic interference (EMI), protecting the avionics fromlightning strikes, dissipating the heat generated by the avionics, andprotecting the avionics from environmental exposure.

Weight is also a consideration for the avionics chassis. The avionicschassis should perform the beneficial functions without unnecessarilyadding weight to the aircraft.

The performance of the beneficial functions is often inapposite tomaintaining or reducing the weight of the avionics chassis, especiallyin light of newer avionics having faster processing speeds and higherfrequencies, smaller size, and greater power densities. These avionicsgenerate relatively large amounts of heat, but operate only under acertain range of threshold temperatures, which leads to an increasedheat-dissipating requirement that has been previously addressed byincreasing the size of the heat sink, leading to an increased weight.

Historically, commercially available avionics chassis are made ofaluminum, which inherently has the desired shielding, heat dissipating,lightning strike protection, and environmental protection benefits.

BRIEF DESCRIPTION OF THE INVENTION

In one embodiment, an avionics chassis comprises a housing having asubstantially thermally non-conductive frame comprising a composite ofcarbon fibers laid up in an epoxy matrix. The housing also includes atleast two walls, at least one of which is a thermally conductive wallcomprising a composite of carbon fibers in a carbonized matrix, and aplurality of spaced, thermally-conductive, card rails provided on the atleast two walls. The at least two walls are mounted to the frame inopposing relationship such that corresponding card rails on the wallsdefine an effective slot therebetween in which a printed circuit boardmay be received and the card rails and the at least one thermallyconductive wall form a thermally conductive path from the interior tothe exterior.

BRIEF DESCRIPTION OF THE DRAWING

In the drawings:

FIG. 1 is a schematic view of an aircraft having an avionics chassisaccording to the invention.

FIG. 2 is a perspective view of the avionics chassis according to oneembodiment of the invention, with a cover removed for clarity.

FIG. 3 is an exploded view of the avionics chassis shown in FIG. 2.

FIG. 4 is a cross-sectional view taken along the line 4-4 of a portionof the avionics chassis shown in FIG. 2.

FIG. 5 is a cross-sectional view taken along the line 5-5 of a portionof the avionics chassis shown in FIG. 2.

FIG. 6 is a cross-sectional view of a portion of the avionics chassishaving an optional card rail mount for the card rails and forming asecond embodiment of the invention.

FIG. 7 is a cross-sectional view taken along the line 7-7 of a portionof the avionics chassis shown in FIG. 2.

FIG. 8 is a bottom view of the thermal plane and stiffener shown in FIG.7.

FIG. 9 is a cross-sectional view of a portion of the avionics chassishaving an alternative thermal plane and thermal pad and forming a thirdembodiment of the invention.

FIG. 10 is a cross-sectional view of a portion of the avionics chassishaving optional attachment structures for the printed circuit boardforming a fourth embodiment of the invention.

FIG. 11 is an exploded view of a fifth embodiment of the avionicschassis according to the invention.

FIG. 12 is an exploded view of a sixth embodiment of the avionicschassis according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically illustrates an aircraft 10 with an on-boardavionics chassis 12 (shown in phantom) for housing avionics for use inthe operation of the aircraft 10. The avionics chassis 12 houses avariety of avionics elements and protects them against contaminants,electromagnetic interference (EMI), radio frequency interference (RFI),vibrations, and the like. While illustrated in a commercial airliner,the avionics chassis 12 can be used in any type of aircraft, forexample, without limitation, fixed-wing, rotating-wing, rocket,commercial aircraft, personal aircraft, and military aircraft. Theavionics chassis 12 may be located anywhere within the aircraft, notjust the nose as illustrated.

FIG. 2 illustrates the avionics chassis 12 according to one embodimentof the invention, with a front cover 42 removed. The avionics chassis 12includes a chassis housing 16 that defines an interior 18 and exterior19. Pluralities of thermally conductive card rails 20 define effectiveslots 21 (illustrated by the dotted lines) there between for receivingprinted circuit boards (PCBs) 14. Mounting feet 22 extend from thechassis housing 16 to facilitate mounting the avionics chassis 12 to theaircraft 10 by means of bolts or other conventional fasteners. Further,the mounting feet 22, can function as an electrical ground for groundingthe avionics chassis to the frame of the aircraft 10. While mountingfeet 22 are shown in this example the avionics chassis 12 can be usedwith any type of attachment mechanism.

FIG. 3 illustrates the avionics chassis 12 and the PCB 14 in moredetail. For purposes of this description, it is noted that the PCB 14may have negative characteristics for an avionics chassis environment,such as heat producing and radio wave sensitivity, which the chassis 12is designed to address. The PCB 14 includes heat producing circuitryand/or at least one heat-producing component 24, such as a semiconductorchip, that is mounted on and supported by a substrate 26, which isgenerally thermally non-conductive. The PCB 14 may be provided withthermally conductive side strips 28 located along the exterior edges ofthe PCB 14. Thermally conductive elements or interior paths 30 may beprovided on the substrate 26 and/or in the interior of the PCB 14. Theinterior paths 30 create a thermally conductive path from the heatproducing component 24 to the thermally conductive side strips 28 toprovide a direct thermal pathway from the interior to the periphery ofthe substrate 26. The side strips 28 then provide a thermal pathway tothe card rails 20. The interior paths 30 can be one or more metalstrips, typically copper, or other conductive material formed in orprovided on the substrate 26.

As illustrated in FIG. 3, the chassis housing 16 comprises a frame 34having a top cover 36, a bottom wall 38, a back wall 40, and opposingside walls 44 and 46, collectively referred to as the walls. The sidewalls 44 and 46 have an interior surface 48 and an exterior surface 50.A plurality of heat-dissipating fins 58 may project from the walls andare illustrated as projecting from the exterior surface 50 of side walls44 and 46. A removable front cover 42 includes openings 47 that may beconfigured for receiving a connector for connecting the one or more PCBs14 to a wire harnesses or the like (not shown).

The frame 34 comprises both polyacrylonitrile (PAN) carbon fibers andpitch carbon fibers in an epoxy matrix. PAN fibers, compared to pitchfibers, have a very high strength and small diameter, which makes themsuitable for use at the various radii of the frame 34. However, PANfibers, compared to pitch fibers, have a low thermal conductivity. Thus,the use of PAN fibers in the frame 34 results in the frame 34 being verystrong, and satisfying the strength requirements for the avionicschassis 12. The frame 34 has an undesirably low thermal conductivity,largely due to an insulative matrix, which is not capable in and ofitself of conducting the heat that is anticipated to be generated by thePCBs 14.

The walls are made with pitch fibers, which have a high stiffness tohelp meet mechanical requirements for the avionics chassis 12. The pitchfibers are not as strong as PAN fibers, so they are more prone tobreaking under stress or during manufacturing. While the walls are notas strong as the frame 34, they need not be because the frame 34provides the primary source of strength for the avionics chassis 12. Theuse of pitch fibers helps reduce wall thickness with no loss instiffness, and the PAN fibers in the frame 34 help maintain mechanicalrequirements. The pitch fibers have a higher thermal conductivity thanthe PAN fibers. Thus, the walls provide more thermal conductivity thanthe frame 34.

The carbon composite has a lower density than traditionally-usedaluminum, which reduces the material weight in the avionics chassis 12while still providing the required strength and stiffness. Because thecomposite has a much lower density, the weight of the chassis housing 16may be reduced a substantial amount. For example, avionics chassis madeaccording to this embodiment have resulted in about a 40% weightreduction. The amount of reduction may vary depending on the mechanicalrequirements for a particular avionics chassis 12.

In forming the frame 34, top cover 36, bottom wall 38, back wall 40, andopposing side walls 44 and 46, a dry lay-up method or pre-preg processof constructing carbon composites may be used with both the pitch andPAN carbon fibers. In such a process, the carbon fiber material isalready impregnated with the epoxy (pre-preg) and may be applied to afemale or male mold. Pre-preg lay-up is a relatively inexpensive, commonprocess that is low cost and well suited for handling thin walled parts.In this embodiment, pre-preg was applied to a female mold.

Bladder molding or other suitable techniques may be used to exertpressure on the pre-preg composite material in the female mold or on themale mold, thereby forcing the composite material to take the shape ofthe mold. Using bladder molding in a female mold the frame 34, backpanel 40, bottom panel 38, and side walls 44 and 46 of the avionicschassis 12 may be formed as an integral unit.

As an alternative to using bladder molding to exert pressure, anelastomeric male mandrel tool may be used. The elastomer expands whenheated to create pressure and consolidate the composite in the femaletool or mold. The heat-dissipating fins 58 may be separated byelastomeric spacers during cure and may thus be co-cured to the sidewalls 44 and 46 to achieve good consolidation, and walls flatness,eliminating seams, and improving thermal paths. Alternatively, theheat-dissipating fins 58 may be formed by machining. Any fittings orposts may be post-bonded to the interior 18.

The top cover 36 and front cover 42 may be produced through compressionmolding with matched metal tooling and may be suitably joined to theframe 34 using any convenient method such as fasteners, solders, brazes,welds, adhesives, and the like. For example, a structural adhesive maybe used to hold the top cover 36 and front cover 42 to the frame 34.Then, to electrically seal the avionics chassis 12, an electricallyconductive adhesive may be placed right next to the structural adhesiveon the interior 18 of the avionics chassis 12.

The card rails 20 abut the interior surface 48 and may be fixedlymounted thereto. The card rails 20 can be attached to the interiorsurface 48 using any convenient method such as mechanical fasteners,solders, brazes, welds, adhesives, and the like. The card rails 20 maybe arranged in pairs, with one card rail 20 residing on the side wall 44and the other card rail 20 residing on the side wall 46 to effectivelydefine a slot 21 extending between the pair of card rails 20.Parallelism between the pair of card rails 20 is necessary to ensurethat the PCB 14 will slide into the slot 21 properly. Each of the cardrails 20 has two legs that define a groove or channel 52, whichpartially defines the slot 21. The card rails 20 should be centered suchthat when the PCB 14 is inserted into the slot 21, the PCB 14 issupported by both of the card rails 20 forming the slot 21; thisfacilitates symmetric cooling of the PCB 14. The card rails 20 may bemade of any suitable thermally conductive material including eithermachined or extruded aluminum, copper, aluminum/beryllium alloy,machined silicon carbide or a metal matrix composite.

A radio wave shield 54 is provided on the housing 16 to render theavionics chassis 12 EMI/RFI resistant. The radio wave shield 54 maycomprise a metallic layer 55 provided on the housing 16. The radio waveshield 54 may be in the form of a metal deposition layer on the chassishousing 16. The deposition layer may be formed by chemical vapordeposition, physical vapor deposition, or electrodeposition. Further,the radio wave shield 54 may be formed by other means such as thermalsprayed metal, the use of a co-cured mesh, or the use of a metal foil.To properly attenuate the electromagnetic interference, the radio waveshield 54 covers or overlies at least the entire exterior of theavionics chassis 12. It may also be applied to the interior if needed.The radio wave shield 54 reflects the radio waves. While the compositematerial of the avionics chassis 12 absorbs some radio waves andprovides some attenuation benefit, the wave shield 54 provides thenecessary attenuation for practical purposes. The contemplated radiowave shield 54 attenuates the radio wave energy by at least 60 dB. Thethickness of the metallic layer 55 for the selected material, isbelieved to be the main factor in attenuating the radio wave energy. Aphysical vapor deposition layer of aluminum having a thickness of 2-3microns has been found to attenuate the radio wave energy at least 60dB.

At least one lightning strike conductive path, comprising a metallicstrip 56, is provided on the chassis housing 16 in addition to theexterior metal layer. The metallic strip 56 is illustrated as overlyingthe metallic layer 55 forming the radio wave shield 54. Whileillustrated as a single metallic strip 56, multiple strips may be usedand it may extend around corners and on multiple components of theassembly. The metallic strip 56 extends to one or more of the feet 22,resulting in the metallic strip 56 forming a conductive path to theelectrical ground. The single metallic strip 56 and/or the multiplemetallic strips may extend to one or multiple feet 22 to providemultiple conductive paths to the electrical ground.

While the mounting feet 22 are illustrated as the grounding point forthe avionics chassis 12. Other suitable grounding points may be used andinclude: grounding studs, grounding surfaces, grounding straps, metallicspring fingers, etc. to provide a grounding path. These may all be donetotally independently of the mounting feet 22. It is contemplated thatthe avionics chassis 12 may not even have mounting feet 22 such as whenmounting hooks and guide pins are used.

It has been contemplated that thermal sprayed aluminum, or anotherthermal sprayed metal, may be used to create the metallic strip 56.Thermal sprayed aluminum is applied by propelling molten aluminum at theavionics chassis 12 with expanding gasses. The molten metal quenches atimpact and adheres to the avionics chassis 12 by mechanical interlockand diffusion bonding. Subsequent impacting aluminum builds the metallicstrip 56 thickness. The metallic strip 56 is relatively thick comparedto the metallic layer 55 of the radio wave shield 54, with a practicalthickness of around 76 microns or greater.

The density and thickness of the metallic strip 56 should be selected toenable the current generated by a lightning strike to be quicklytransmitted to the electrical ground without causing electro-migrationor the fusing of the metallic strip 56. FIG. 4 illustrates a crosssection of the metallic layer 55 and metallic strip 56 located onseveral of the heat-dissipating fins 58. The thickness of the metallicstrip 56 is shown schematically as being thicker than the thickness ofthe metallic layer 55.

The thermal sprayed aluminum may also be applied over bonded joints onthe avionics chassis 12. For example, where the mounting feet 22 areattached to the chassis housing 16. The thermal sprayed aluminum, ormetallic strip 56, creates a continuous, intimately bonded conductivepath between the chassis housing 16 and the mounting feet 22 and thishelps to avoid slight gaps between the conductive paths, which couldenable sparking. The electrical resistance between any locations on theavionics chassis 12, including the mounting feet 22, may not exceed 2.5milliohms.

Unlike its metal counterparts, the carbon composite avionics chassis 12does not inherently attenuate radio wave energy or conduct away theextreme electrical currents generated by lightning strikes. This isbecause the carbon fiber composite chassis housing 16 is significantlyless electrically conductive than an aluminum chassis because of anelectrically insulative composite matrix. In a carbon fiber compositeavionics chassis 12 current from a lightning strike seeks the metalpaths available, which can damage and even destroy onboard electronicsthat have not been electromagnetic field shielded or lightningprotected. The metallic layer 54 described above is not always thickenough to handle a lighting strike. Also, a thick enough metal layer toprovide lightning strike protection greatly and unnecessarily increasesthe weight of the avionics chassis 12.

The combination of different materials and thicknesses for the metalliclayer 55 and metallic strip 56 provide for additional weight reduction,while still providing the desired radio wave shielding and lightningstrike protection. The mixing of the metallic layer 55 and metallicstrip 56 along with limiting their respective coverage area to thatrequired to perform the desired function provides for a substantialweight reduction.

FIG. 5 illustrates that the card rail 20 may be attached to the interiorsurface 48. The card rail 20 may be attached using fasteners, solders,brazes, welds, adhesives, or other attachment methods. If a structuraladhesive is used, it will not have the necessary electrical conductivityand thus thermal sprayed aluminum, another thermal sprayed metal, or ametal applied by another means may be applied along the card rail 20 toincrease electrical conductivity between the card rail 20 and theinterior surface 48 of the side walls 44 and 46.

The plurality of heat-dissipating fins 58 extend from the exteriorsurface 50 of the side walls 44 and 46. Because the carbon fiber in theavionics chassis 12 is encased in the epoxy matrix, the resultingstructure has the structural and weight benefits of the carbon fiber butnot the thermal conductivity benefits. In this embodiment, the sidewalls 44 and 46 are integrated cold walls that help create a thermalmanagement system to conduct heat from the interior 18 of the avionicschassis 12 to its exterior 19 where the heat may then be dissipated inthe surrounding air through convection.

While other configurations are possible, the heat-dissipating fins 58are illustrated in FIGS. 2 and 5 as having the same orientation andcommensurate in length to the card rails 20. For example, theheat-dissipating fins may run perpendicular to the card rails. Theheat-dissipating fins 58 increase the exterior surface area of the sidewalls 44 and 46 allowing more heat to be transferred to the surroundingair through convection. The heat-dissipating fins 58 are schematicallyillustrated in FIGS. 4 through 6 as comprising a plurality ofhigh-thermal conductivity carbon fibers 59 with isotropic orientation inthe plane of the heat-dissipating fins 58. The use of the orientedcarbon fibers gives the heat-dissipating fins 58 several times thethermal conductivity, yet significantly less weight, than an aluminumpart of similar dimensions. For example, the isotropic carbon fibers 59can have a high-thermal conductivity of approximately 1100 W/m-K.

The heat-dissipating fins 58 can be co-cured to the side walls 44 and 46eliminating seams and improving thermal paths. To further improvethermal conductivity, a plurality of isotropic fibers of theheat-dissipating fins 58 may be extended at discrete sites from aninterior of the heat-dissipating fins 58 to create tabs 60. These tabs60 may be formed along the entire length of the heat-dissipating fin 58.The tabs 60 go through the side walls 44 and 46 to contact the cardrails 20 located on the interior surface 48. The isotropic carbon fibers59 form a direct conductive path from the card rail 20 to theheat-dissipating fins 58.

Not all of the heat-dissipating fins 58 in abutting contact with theexterior surface 50 have tabs 60 extending through the side walls 44 and46 to the card rail 20. The plurality of isotropic carbon fibers 59extending from the heat-dissipating fins 58 through the side walls 44and 46 and in abutting contact with the card rail 20 is advantageoussince it significantly improves heat transfer. Multiple tabs 60 from oneheat-dissipating fin 58 may contact the card rail 20 down its entirelength. Further, a plurality of tabs 60 from a plurality ofheat-dissipating fins 58 are illustrated as abutting the single cardrail 20 this also improves the amount of heat that may be conducted fromthe card rail 20.

FIG. 6 illustrates an alternative mounting of the card rails 20. Morespecifically a card rail mount 61 is provided on the card rail 20 andattached to the interior surface 48. The card rail mount 61 isillustrated as a pedestal 62 having a grooved surface 64. The card railmount 61 may be adhered by at least one of a structural adhesive and aconductive adhesive to the card rail 20. Depending on the application,the same adhesive may provide both the desired structural and conductiveproperties.

The grooved structure 64 defines intervening interstitial spaces 65 thatmay receive thermally conductive adhesive 67 when the card rail mount 61is adhered to the interior surface 48. This thermally conductiveadhesive may touch the isotropic carbon fibers 59 to help form aconductive path from the card rail 20 to the heat-dissipating fins 58.Additionally, a plurality of fasteners 66, such as screws, may beinserted into the exterior surface 50 to provide mechanicalreinforcement and ensure stability of the card rails 20.

FIG. 7 illustrates a portion of the avionics chassis 12 including acircuit card assembly 68, mounted in the slot 21, and having a thermalplane 70, a thermal pad 76, and stiffeners 78. The PCB 14 is illustratedas being mounted within the slot 21 with a thermal plane 70 also in theslot 21 and in overlying relationship with the PCB 14. In this manner,the PCB 14 defines a first primary plane, the thermal plane 70 defines asecond primary plane, and the spatial relationship between the PCB 14and the thermal plane 70 is such that the first and second primaryplanes are located within the slot 21 when the circuit card assembly 68is received within the slot 21.

FIG. 8 better illustrates the thermal plane 70, the thermal pad 76, andthe stiffeners 78. The thermal plane 70 is used to conduct heat awayfrom the PCB 14. The thermal plane 70 can be comprised of a carbonfiber-reinforced composite as well as a carbon-carbon composite. Forexample, the thermal plane 70 may be comprised of pyrolytic carbon,which is highly thermally conductive. The carbon fibers may be laid upsuch that the thermal plane 70 is thermally conductive in thetwo-dimensional plane, that is it has in-plane (lateral) thermalconductivity that enables heat to dissipate in the x and y plane. It isalso possible for the thermal plane 70 to have a lay-up of carbon fibersin 3D. The 3D lay-up would be more expensive but would facilitate themovement of heat away from the PCB 14. It has been contemplated that aone-dimensional lay-up may also be useful. No matter its configuration,the thermal plane 70 is intended to thermally conduct heat from the PCB14 towards the card rails 20.

The thermal plane 70 may be attached to either the top or the bottom ofthe PCB 14. The thermal plane 70 may be mounted directly to the PCB 14or through the thermal pad 76. The thermal pad 76 may be made of acarbon composite or any thermally conductivity material. For example,the thermal pad 76 may be made from 3D carbon-carbon composite. Thethermal pad 76 may be located such that it directly contacts theheat-producing component 24.

The stiffener 78 is operably coupled to the PCB 14 so that the PCB 14will not flex or vibrate within the slot 21. The stiffener 78 can belocated between the PCB 14 and the thermal plane 70 when the circuitcard assembly 68 is located within the slot 21. The stiffener 78 canalso be located within one of the card rails 20 when the circuit cardassembly 68 is located within the slot 21. The stiffener 78 may becomprised of aluminum or similar thermally conductive material and canhave a variety of configurations to provide support for the PCB 14.Although the thermal plane 70 has been illustrated as a plane, it hasbeen contemplated that it may also be a bar or a strap. Furthermore, inalternate embodiments, any suitable shape stiffener 78 for strengtheningthe PCB 14 could be provided. For example, the stiffener 78 may beseveral bars that are not interconnected. The stiffener 78 can also beintegral with the thermal plane 70.

Referring back to FIG. 7, when the circuit card assembly 68 is in theslot 21, the thermal plane 70 is conductively coupled to one of the cardrails 20 to form a portion of a first conductive path 72 and the PCB 14is conductively coupled to another of the card rails 20 to form aportion of a second conductive path 74. The first conductive path 72begins with the heat-producing component 24; heat is conducted throughthe thermal pad 76 to the thermal plane 70, which in turn conducts thatheat laterally to the card rails 20. The first conductive path 72continues through the card rails 20 to either the isotropic carbonfibers 59 in the tabs 60 or the side walls 44 and 46 themselves. Theheat conducted through the isotropic carbon fibers 59 in the tabs 60 isdirectly conducted to the exterior of the heat-dissipating fins 58. Theheat conducted through the side walls 44 and 46 is conducted by theisotropic carbon fibers 59 in the heat-dissipating fins 58 to theexterior of the heat-dissipating fins 58. Heat may then be dissipatedthrough convection into the air surrounding the heat-dissipating fins58.

The second conductive path 74 begins with the heat-producing component24; heat is then transferred through the interior paths 30 of the PCB 14to the thermally conductive side strips 28. Although the arrowsillustrated in FIG. 7 are offset from the interior paths 30, this isdone for illustrative purposes and the interior paths 30 are actually aportion of the second conductive path 74. The arrow has merely beenoffset so that it does not obscure the interior paths 30 in the figure.The side strips 28 abut the card rail 20 and heat in turn conducts fromthe card rail 20 either through the side walls 44 and 46 to the exteriorof the heat-dissipating fins 58 or through the tabs 60 to the exteriorof the heat-dissipating fins 58. Heat may then be dissipated throughconvection into the air surrounding the heat-dissipating fins 58. Thus,the PCB 14 also acts as a heat spreader by itself. This allows theavionics chassis 12 to run much cooler with the additional conductivepath provided by the thermal plane 70.

The height of the PCB 14 is such that the PCB 14 and thermal plane 70are both received within the channel 52. As illustrated in FIG. 7, thePCB 14 is in direct contact with the main portion of the card rail 20.The thermal plane 70 is in direct contact with the leg of the card rail20 and in direct contact with the main portion of the card rail 20.Alternatively, the contact between the PCB 14 and the card rail 20 orthe contact between the thermal plane 70 and the card rail 20 could beindirect contacts.

FIG. 9 illustrates an alternative thermal pad comprising an adjustablethermal pad 80. The adjustable thermal pad 80 is illustrated as a screwcontact 82. The lower portion of the screw contact 82 is adjustablerelative to the PCB 14. Thus, the screw contact 82 may be lowered andraised such that it may accommodate heat-producing components 24 ofvarying heights.

FIG. 10 illustrates an alternative mounting of the PCB 14 in the cardrails 20. More specifically, wedge locks 79 may be used to connect thePCB 14 and the thermal plane 70 to the card rails 20. The wedge locks 79may be made of aluminum or any similarly thermally conductive material.In this manner, the wedge locks 79 may become a portion of the firstconductive path 72 and the second conductive path 74. For example, thesecond conductive path then begins with the heat-producing component 24;heat is then transferred through the interior paths 30 to the thermallyconductive side strips 28. The side strips 28 abut the wedge locks 79,which in turn conduct heat to the card rail 20. The card rail 20 in turnconducts heat through the side walls 44 and 46 to the heat-dissipatingfins 58. Heat may then be dissipated through convection into the airsurrounding the heat-dissipating fins 58. Again, although the arrowsillustrated in FIG. 9 are offset from the interior paths 30, this isdone for illustrative purposes and the interior paths 30 are actually aportion of the second conductive path 74.

With either embodiment, the height of the components of the circuit cardassembly 68 should be selected such that the entirety of the circuitcard assembly 68 is located within the slot 21. This gives the circuitcard assembly 68 a low profile design. This will allow more circuit cardassemblies 68 to be placed in the avionics chassis 12. As the amount ofcircuit card assemblies 68 in the avionics chassis 12 increases thepresence of the two thermally conductive paths 72 and 74 will helpprovide additional heat distribution from the PCB 14 and allow theavionics chassis 12 to run cooler.

FIG. 11 is an exploded view of an avionics chassis 112 having cold walls144 and 146 according to a fifth embodiment of the invention. The fifthembodiment 100 is similar to the first embodiment 10. Therefore, likeparts will be identified with like numerals increased by 100, with itbeing understood that the description of the like parts of the firstembodiment applies to the second embodiment, unless otherwise noted.

One difference between the first embodiment 10 and the fifth embodiment100 is that the cold walls 144 and 146 are discrete in that they areformed separately from the frame from a thermally conductive material.More specifically, the cold walls 144 and 146 are formed from acomposite of carbon fibers in a carbonized epoxy matrix. Carbonizedepoxy matrix composites have relatively high thermal conductivityproperties in each axes compared to epoxy matrix composites; thermalconductivity is increased in the axes depending on the carbon fiberlay-up. The carbon fibers in the cold walls 144 and 146 are laid up suchthat the cold walls 144 and 146 are more thermally conductive in atwo-dimensional plane. The carbon fibers in the carbonized matrix haveexcellent thermal properties in the x and y plane due to the fiberlay-up much like the thermal planes described above.

This configuration provides that the cold walls 144 and 146 may beformed from a higher thermal conductivity material than the remainder ofthe avionics chassis 112 and frame 134. The high thermal conductivity ofthe cold walls 144 and 146 results in the cold walls 144 and 146 beingstiff but not strong. To make a whole avionics chassis out of the samematerial would require the whole avionics chassis 112 to be very thickto achieve the structural support necessary. Thus, the substantiallythermally insulative frame 134 formed from carbon fibers laid up in anepoxy matrix gives the avionics chassis 112 its strength and thediscrete cold walls 144 and 146 can provide the benefits of high thermalconductivity while not being required to provide such rigorousstructural support.

Another difference is that card rails 120 are integrally formed on theinterior surface 148 of the cold walls 144 and 146. The cold walls 144and 146 are mounted to the 134 frame in opposing relationship such thatcorresponding card rails 120 on the cold walls 144 and 146 define a slot121 therebetween. Thus, the cold walls 144 and 146 should be alignedperfectly such that the circuit card assemblies may fit within the slots121. The discrete cold walls 144 and 146 may be assembled to the frame134 using soldering, welding, brazing, adhesive, mechanical fasteners,or other similar attachment methods. Structural adhesive may be appliedto fix the cold walls 144 and 146 to the frame 134 and an electricallyconductive adhesive may be placed right next to the structural adhesiveon the interior 118 of the avionics chassis 112 to electrically seal it.The cold walls 144 and 146 may also be metal plated, such as with nickelor aluminum, to provide better conductivity and to seal the carbonfibers against galvanic corrosion with aluminum wedge locks 179 on thePCBs 114.

FIG. 12 is an exploded view of an avionics chassis 212 having cold walls244 and 246 according to a sixth embodiment of the invention. The sixthembodiment 200 is similar to the fifth embodiment 100. The differencebeing that the cold walls 244 and 246 include heat-dissipating fins 258to increase the surface area of the exterior surface 250 of the coldwalls 244 and 246. The cold wall surface area may also be increased withpins or other similar methods.

From a weight perspective, a carbon fiber composite avionics chassis 12is more desirable than a heavier aluminum version. However, the carbonfiber composite version is less desirable than an aluminum versionbecause of the poorer thermal and electrical conductivitycharacteristics. Thus, the various embodiments of carbon fiber compositeavionics chassis disclosed herein are beneficial for an aircraftenvironment because of their weight reduction. The reduced weightavionics chassis also addresses all requirements related toelectromagnetic interference (EMI), dissipating the heat generated bythe avionics, protecting the avionics from lightning strikes, andprotecting against environmental exposure, while still achieving arelatively low weight avionics chassis.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

1. An avionics chassis assembly comprising: a housing having an interiorand exterior, comprising: a substantially thermally non-conductive framecomprising a composite of carbon fibers laid up in an epoxy matrix; atleast two walls, at least one of which is a thermally conductive wallcomprising a composite of carbon fibers in a carbonized matrix; aplurality of spaced, thermally-conductive, card rails provided on the atleast two walls; wherein the at least two walls are mounted to the framein opposing relationship such that corresponding card rails on the wallsdefine an effective slot therebetween in which a printed circuit boardmay be received and the card rails and the at least one thermallyconductive wall form a thermally conductive path from the interior tothe exterior.
 2. The avionics chassis assembly of claim 1, furthercomprising at least one printed circuit board located within the slot.3. The avionics chassis assembly of claim 2, wherein the card railscomprise grooves forming part of the slot and the printed circuit boardcomprises wedge locks that are received within the grooves toselectively lock the printed circuit board to the card rails and formpart of the thermally conductive path.
 4. The avionics chassis assemblyof claim 3, wherein the wedge locks are metal and the walls are metalplated to seal the carbon fibers against galvanic corrosion with thewedge locks.
 5. The avionics chassis assembly of claim 4, furthercomprising heat-dissipating fins composed of carbon fiber and projectingfrom the at least one thermally conductive wall to the exterior of thehousing to form part of the thermally conductive path.
 6. The avionicschassis assembly of claim 5, wherein the heat-dissipating fins areco-cured to the housing.
 7. The avionics chassis assembly of claim 5,wherein the heat-dissipating fins are formed by machining.
 8. Theavionics chassis assembly of claim 6, wherein the heat-dissipating finsare aligned with the rails.
 9. The avionics chassis assembly of claim 8,wherein the heat-dissipating fins are coextensive with the rails. 10.The avionics chassis assembly of claim 1, wherein the walls are mountedto the frame with at least one mechanical fastener.
 11. The avionicschassis assembly of claim 1, wherein the walls are mounted to the framewith at least one adhesive.
 12. The avionics chassis assembly of claim11, wherein the at least one adhesive is thermally conductive.
 13. Theavionics chassis assembly of claim 11, wherein the at least one adhesiveis a structural adhesive.
 14. The avionics chassis assembly of claim 1,wherein the carbon fibers in the at least one thermally conductive wallare laid up such that the wall is thermally conductive in atwo-dimensional plane.
 15. The avionics chassis assembly of claim 1,wherein the walls are metal plated.
 16. The avionics chassis assembly ofclaim 1, wherein the housing further comprises at least one thermallynon-conductive wall.